The present invention relates to a glow plug used to preheat a subcombustion or combustion chamber of a diesel engine, and more particularly, to an improvement in a self-temperature control type glow plug with a rod heater for achieving a fast heating function and improving heating characteristics to achieve a prolonged after-glow.
Since a diesel engine generally has poor starting characteristics at low temperatures, a glow plug is mounted in a subcombustion or combustion chamber thereof. A current is supplied to the glow plug to heat it. The heat from the glow plug increases an intake temperature, or is used as an ignition source, so that the starting characteristics of the diesel engine are improved. A typical conventional glow plug is of a sheath type wherein a metal sheath is filled with a refractory insulating powder, and a coil heater of iron chromium, nickel or the like is embedded in the powder. Another typical conventional glow plug is of a ceramic heater type, described in Japanese Patent Prepublication No. 57-41523. The ceramic heater type glow plug has a rod heater prepared by embedding a heater wire of tungsten or the like in a ceramic material. In this type of plug, when compared with the sheath type glow plug which performs indirect heating through the refractory insulating powder and the metal sheath, heat conduction efficiency is improved as are heat conduction characterisitics. The ceramic heater type glow plug need only be heated for a short period of time, thereby improving temperature rise characteristics and satisfying fast-heating glow plug requirements to some extent. Because of these advantages, ceramic heater type glow plugs have become popular in recent years.
Still another conventional glow plug is proposed, as a self-temperature control type glow plug with two heating materials, in Japanese Patent Publication No. 45-11648 and Japanese Patent Prepublication No. 54-109538. In this type of plug, a resistor with a larger positive temperature coefficient (PTC) than the conventional heating wire is connected as an energization power control element in series with the heating wire of the glow plug. Energization power to the heating wire is self-controlled to greatly improve the heating characteristics and prevent overheating of the heater portion of the plug.
However, since only one type of heating wire is embedded inside the conventional ceramic heater glow plug in the same manner as in the conventional sheath type glow plug, some problems occur in energization power control. In order to greatly improve the temperature rise characteristics upon heating of such a ceramic heater, a large current must be supplied to the heater in the initial energization period to quickly heat the heating wire. In this case, the heating wire becomes fused and disconnected, and residual high heat adversely affects the ceramic heater. Therefore, when one type of heating wire is used, power control cannot be satisfactorily performed, thus resulting in poor fast-heating characteristics. Furthermore, fast heating also adversely affects the battery and the electrical circuit. In the worst case, fuses therein are disconnected.
In order to prevent the above problems, a temperature control means must be additionally arranged on the heating wire circuit. As a result, the preheating device including the glow plug inevitably results in high cost. In such a ceramic heater type glow plug, a heating wire of tungsten or the like is embedded in a ceramic heater of silicon nitride or the like. A temperature profile within the heater tends to be nonuniform, reliability of heat-resistance is degraded, and the manufacturing cost is very high. For these reasons, a proper countermeasure must be provided.
A conventional self-temperature control type glow plug with two coils might solve the problems presented by the conventional ceramic heater type glow plug, but some problems on the reliability of the control function and the like are present. In addition, some problems with fast heating, since the self-temperature control type glow plug performs indirect heating through the ceramic material in the same manner as in the sheath type glow plug, are also present. More specifically, in the conventional self-temperature control type glow plug with two different heating materials, a power control resistor is mounted in a holder while the holder is filled with an insulating material such as soluble glass. The power control resistor is connected in series with a heating wire in a sheath. This type of plug has a complex structure, and its assembly is time-consuming and complicated. In addition, it is difficult to properly control the function of the resistor. With such a structure, it is impossible to shorten the desired temperature rise to a period within 7 seconds. Along with the prolongation of a period of energization, i.e., so-called after-glow upon starting of the engine, even though such prolongation is the latest market demand, degradation of the wire material itself is a critical problem.
In particular, in the conventional glow plug of this type, demand has recently arisen for an "after-glow" system which improves starting characteristics of diesel engines and has durability against high-temperature operating conditions required by the widespread use of turbo chargers, and which reduces the cost of the preheating device as a whole. According to such an after-glow system, the energization state of the glow plug is maintained for a predetermined period of time after starting the engine, so that combustion in the engine can be smoothly and properly performed and hence the amount of exhaust gas and noise can be decreased. In order to do this, the after-glow time must be as prolonged as possible. Even after the engine is started, it is usually too cold to be operated in a normal state in cold environmental temperatures, and takes a long period of time to attain a proper idling state. Furthermore, when the engine is cold, idling noise is increased and white smoke is exhausted, due to incomplete fuel combustion. In the worst case, the engine stops. Therefore, the after-glow effect is required to prevent these problems.